The present invention pertains to valves and in particular to tri-leaflet heart valve prostheses.
Ever since 1950, when blood oxygenators made open heart surgery feasible, it has been possible to treat some forms of heart disease by replacing one of the patient""s heart valves with a prosthetic valve. Early heart valve prostheses included ball-and-cage valves in which a ball or disc was housed in a cage. One side of the cage provided an orifice through which blood flowed either into or out of the heart, depending on the valve being replaced. When blood flowed in a forward direction, the energy of the blood flow forced the ball or disc to the back of the cage allowing blood to flow through the valve. When blood attempted to flow in a reverse direction, or xe2x80x9cregurgitatexe2x80x9d, the energy of the blood flow forced the ball or disc into the orifice in the valve and blocked the flow of blood.
Bi-leaflet heart valves were developed to overcome some of the deficiencies of early cage-based designs. A bi-leaflet valve comprises an annular valve body in which two opposed leaflet occluders are pivotally mounted. The occluders are typically substantially rigid, although some designs incorporate flexible leaflets, moving between a closed position in which the two leaflets are mated to prevent blood flow in the reverse direction, and an open position in which the occluders are pivoted away from each other to permit blood flow in the forward direction. The energy of blood flow caused the occluders to move between their open and closed positions.
Tri-leaflet heart valves are another type of valve developed to overcome deficiencies of prior valve designs. A tri-leaflet valve comprises an annular valve body in which three flexible leaflets are mounted to a portion of the valve body, called a xe2x80x9cstentxe2x80x9d, located at the circumference of the annulus. Although some tri-leaflet valves use rigid leaflets, flexible leaflets are typical. When blood flows in the forward direction, the energy of the blood flow deflects the leaflets away from the center of the annulus and allows blood to flow through the valve body in the forward direction. When the pressure across the valve reverses and blood begins to flow in the reverse direction, the three leaflets engage each other in a coaptive region, occluding the valve body annulus and preventing the flow of blood through the valve in the reverse direction. The valve leaflets may be made from tissue, such as specially treated porcine or bovine pericardial tissue, or from man-made materials such as polyurethane, silicone rubber or other biocompatible polymer.
One of the issues considered in the design of heart valves incorporating flexible leaflets is the durability of the leaflets. When the valve is in a xe2x80x9cclosedxe2x80x9d position (i.e., when it is closed and may be under a reverse pressure load) the leaflets experience stress. In addition, the valve commissure, which is the region where the leaflets contact the attachment curve and an adjacent leaflet, also experience high mechanical stress. The commissure is also the location of physical characteristics, called xe2x80x9cstress risers,xe2x80x9d which increase the amount of stress on the leaflet near the commissure. For example, an edge of a tissue leaflet that has been cut by a knife blade often adjoins or is near the commissure. The trauma of being cut by a blade creates stress risers along the cut edge. Further, the commissure is an area where the flexible leaflet is coupled to the less flexible valve body, which create stress risers. Finally, the commissure has small radius corners that create stress risers.
Prior heart valve designs have incorporated flexible stents into the valve body to reduce the mechanical stress at the commissure. Another prior art approach to reducing mechanical stress is to incorporate non-isotropic materials, such as fabric, into the leaflets.
The present invention is directed to a prosthetic heart valve with flexible leaflets. The leaflets have an analytic shape in a selected position which can be represented generally by analytic geometry. An analytic shape may include a portion of a cylindrical surface, of an ellipsoid, of a paraboloid, or of another shape that can be defined mathematically. The leaflets are coupled to a valve body at an attachment curve. The attachment curve is configured to distribute the stress experienced by the leaflet along the attachment curve when the leaflet is in a selected position. An annular valve body has a non-circular inner wall at least at the attachment curve, the shape of the inner wall being defined by the attachment curve. The invention increases the durability of a leaflet coupled to a valve body by moving the leaflet""s point of maximum loaded stress along the attachment curve away from the location of stress risers.
The valve may further comprise a leaflet coupled to the leaflet support; the shape of the leaflet support being configured to locate a point of maximum stress on the leaflet away from a location of stress risers on the leaflet.
The invention may have a valve comprising a valve body and a first leaflet coupled to the valve body along an attachment curve. The attachment curve is configured to locate a point of maximum loaded stress on the first leaflet away from a location of stress risers on the first leaflet. The invention may also comprise a second leaflet and a commissure between the first leaflet and the second leaflet wherein the location of stress risers is the commissure.
The invention may also comprise a valve having a valve body and a leaflet coupled to the valve body along an attachment curve. The leaflet may comprise a surface and a free edge, the free edge comprising a center point. A region of maximum loaded stress may comprise a range of points along the attachment curve that are substantially an equal distance, measured along the surface of the leaflet, from the center point of the leaflet""s free edge. The region of maximum loaded stress may be closer, measured along the surface of the leaflet, than any point along the attachment curve that is not among the points in the region of maximum loaded stress.
In general, in another aspect, the invention features a method of making a valve. An analytic shape for one or more flexible leaflets in a selected position is selected. An attachment curve for a leaflet is selected. Because the leaflet has an analytic shape, mathematical modeling (i.e., finite element analysis) may be used to predict stress at the attachment curve. The shape of the attachment curve is revised until predicted stress falls within acceptable design limits. A valve body is formed having a longitudinal axis in a direction of blood flow. An inner wall of the valve body is shaped to conform to the attachment curve. The leaflet is coupled to the valve body at the attachment curve.
In another aspect, the invention features a method of making a valve comprising selecting a leaflet having an analytic shape in a selected position. An attachment curve is selected. The attachment curve has a first end and a second end. The shortest distance along a surface of the leaflet from a center of a free edge of the leaflet to a point of maximum stress along the attachment curve is less than the distance along the surface of the leaflet from the center of the free edge of the leaflet to the first end of the attachment curve. A valve body is formed. The valve body has an inner wall conforming to the attachment curve and coupling the leaflet to the valve body along the attachment curve.
The invention may feature a valve comprising a valve body and a leaflet coupled to the valve body along an attachment curve. The attachment curve comprises a first end and a second end. The leaflet is movable between an open position and a closed position. The leaflet has a point of maximum displacement between the open and closed positions. The shortest distance along the surface of the leaflet from the point of maximum displacement to a point of maximum stress along the attachment curve is less than the distance along the surface of the leaflet from the point of maximum displacement to an end of the attachment curve. The point of maximum displacement may be a center of a free edge of the leaflet.
These and other features and advantages of the invention will be apparent from the following description and accompanying drawings.